LAS VEGAS - The Republican presidential debates have become appointment television. Tens of millions of Americans have tuned in to watch Donald Trump whack his unwieldy field of opponents -- pooh-poohing Jeb Bush's energy level, for example, or disparaging Carly Fiorina's appearance.

So what happens if the biggest fireworks in the first Democratic debate here on Tuesday night are over which candidates opposed the Keystone XL pipeline first, which would resurrect the Glass-Steagall Act or how each would pay for his or her higher education overhauls?

Democrats expect the debate to be substantive and to set the course for an unexpectedly contentious nominating contest. Americans are either going to find a pleasing contrast to the rip-roaring show Republicans have put on -- or they're going to be bored senseless.

"Let's be honest: Donald Trump truly is a ratings machine. Twenty-three million people did not tune in to see Marco Rubio," said Paul Begala, a Democratic strategist and analyst on CNN, which is airing Tuesday's debate. "So, unless the Democrats can talk one of the Kardashians into running, don't expect the Democrats' ratings to approach the Republicans'."

For good or bad, Republicans have engaged a massive swath of the country with their first two debates, which were watched by 25 million and 23 million people respectively. (By comparison, the highest-rated Democratic presidential debate in 2008, when Barack Obama and Hillary Rodham Clinton were locked in a battle royale, drew 10.7 million viewers.) Even if some voters only tuned in to witness the spectacle that is Trump's front-running candidacy, they now are familiar with many of the other GOP candidates.

A test for Democrats is whether Clinton, a former secretary of state and first lady, and a cast of four challengers, led by liberal insurgent Sen. Bernie Sanders (I-Vt.) can generate the same level of interest in their primary campaign.

In the run-up to Tuesday's debate, there are signs that they could. On Sunday, there were 124,812 mentions of the Democratic candidates on social media, television and in newspapers. By comparison, there were 377,223 mentions of the Republican candidates two days out from their Sept. 16 debate, according to an analysis by Zignal Labs, The Washington Post's analytics partner.

However, the Republican statistic is larger in part because it included many more candidates, according to Zignal. Clinton and Sanders consistently have a larger social media presence than any GOP candidate besides Trump.

"More people are talking about the Democrats than about the Republicans, outside of Trump," said Josh Ginsberg, Zignal's co-founder and chief executive. "The Clinton-Sanders horse race is interesting to people."

As they prepare for their first face-off, Clinton and Sanders have signaled that they will wear velvet gloves. Each plans to focus on his or her own policy proposals and backgrounds, drawing comparisons with each other's wherever appropriate but avoiding the kind of direct, personal attacks that have been so prominent in the Republican race.

None of the Democratic candidates is Trump's equal in bombast and showmanship. And unlike many of the other Republican candidates, the Democrats are downright predictable.

Sanders will probably bring the most passion, as he has all summer and fall, drawing massive crowds across the country. Clinton, who earned a reputation as a steady and skilled debater in her 2008 campaign, is by her nature cautious and likely will display more competence than entertainment value.

Two lesser-known candidates, former Virginia senator Jim Webb and former Rhode Island governor Lincoln Chafee, tend to be understated.

"The level of humility and seriousness in this debate will be stark compared to what has been happening in the Republican debates, and that's a good thing for our party," said Stephanie Cutter, a Democratic strategist and veteran of President Barack Obama's campaigns.

The wild card could be former Maryland governor Martin O'Malley, who is struggling to break out of the low single-digits in polls and sees the fall debates as make-or-break opportunities. On the campaign trail, he has fired shots at Clinton, and he could do so on the debate stage.

Still, Anderson Cooper, the CNN anchor and moderator of Tuesday's debate, said he does not anticipate the Democratic candidates will be willing to criticize each other too much. He is preparing for a different kind of debate than the Republicans have held.

"I'm always uncomfortable with that notion of setting people up in order to kind of promote some sort of a face-off," Cooper said Sunday on CNN's "Reliable Sources." "I think these are all serious people. This is a serious debate. They want to talk about the issues. And I want to give them an opportunity to do that."

The debate, to be held at the Wynn Las Vegas luxury resort, will last two hours and begin at 8:30 p.m. ET. There are expected to be five candidates on stage. Should Vice President Joe Biden enter the race before Tuesday night, he is pre-qualified to appear at the debate. His advisers have said he will not attend, although CNN has a sixth podium at the ready in case he springs a surprise.

Harvard professor Lawrence Lessig, who says he has raised more than $1 million from a devoted base of supporters but is polling at or below 1 percent, was not invited.

The Republicans, who began the debate season with 17 candidates (Wisconsin Gov. Scott Walker and former Texas governor Rick Perry have since dropped out), have had to hold undercard debates just to fit everyone on stage.

Steve Schmidt, a strategist on George W. Bush and John McCain's presidential campaigns, said the comparison between what he considers a vibrant Republican debate stage and the Democratic stage of five will not be favorable for the Democrats.

"The country's not yet had an opportunity to see the current field of Democrats all on the stage together," Schmidt said. "Certainly there is a high likelihood that the weaknesses of that field will become more apparent than they are today."